A. Field of the Invention
This invention relates generally to the field of devices used to measure blood pressure. More particularly, the invention relates to a wearable, non-invasive device for accurate and continuous blood pressure data acquisition. The device uses optical techniques for generating blood pressure data. The device either generates and displays the blood pressure data locally, or transmits the data via wireless techniques to a base unit for display or transmission to appropriate monitoring equipment.
B. Statement of Related Art
Non-invasive systems for continuous monitoring of blood pressure, for example during anesthesia, have been proposed in the prior art. Representative patents include the patents to Shinoda et al., U.S. Pat. No. 5,165,416; the patents to Erkele et al., U.S. Pat. Nos. 4,802,488 and 4,799,491; Jones et al., U.S. Pat. No. 5,140,990, Jackson et al., U.S. Pat. No. 5,485,848 and Pytel et al., U.S. Pat. No. 5,195,522. It is also known to use optical sensors as the means to acquire blood pressure data. See the patents to Butterfield, et al., U.S. Pat. Nos. 5,908,027; 5,158,091; 5,261,412 and 5,273,046; Cerwin, U.S. Pat. No. 5,984,874 and Tenerz et al., U.S. Pat. No. 5,018,529. The above-referenced patents are incorporated by reference herein.
Prior art mechanical sensors commonly measure blood pressure by detecting transducer changes that are proportional to the detected changes in external force measured at the skin surface during pulsation. These sensors depend on mechanical parts and are therefore more prone to breakdown due to moving parts, and are larger in size thus requiring more space for fitting it on the patient skin. These sensors employ the use of a single sensor, or an array of sensors from which only one (the one with the highest signal strength) is selected for measurement. Such sensors only cover a small surface area on the skin and are therefore very sensitive to initial exact placement of the sensor on top of the artery. They are also sensitive to movement or minor accidental repositioning. This typically invalidates all calibrations, requiring a need for re-calibrating the system with an air cuff pressure reference. Providing a corrective feedback mechanism for compensating for minor positional changes in sensor placement is not possible due to dependency on a single-point or single-sensor measurement. Furthermore, the resolution of these sensors to blood pressure changes at low level signal strength is not sufficient to obtain accurate results. Other sensors typically require higher hold down pressure (HDP) values in order to obtain a stronger signal strength due to their low sensitivity. They also offer no corrective feedback mechanism for compensating for minor variations in the hold down pressure, often requiring a need for re-calibration of the sensor at the new hold down pressure value.
Portable oscillometric wrist mounted blood pressure devices also exist, such as the Omron model HEM-609, but these are not intended for continuous blood pressure monitoring. The oscillometric method requires the patient to be at a rested state, and a cuff pressure to be applied by the device that is above the systolic blood pressure of the patient (thus temporarily cutting off circulation in the artery and causing discomfort).
Spacelabs"" Modular Digital Telemetry system offers an ambulatory blood pressure (ABP) option for wireless transmission of noninvasive blood pressure data to a central computer, however it is not a tonometric optical blood pressure monitor and it is transmit only.
The above-referenced ""027 Butterfield et al. patent describes a device and technique for measuring tonometric blood pressure non-invasively using a one-dimensional optical sensor array. The sensor used in the ""027 patent is also described in U.S. Pat. No. 5,158,091 to Butterfield et al. The array detects photo-radiation (i.e., light) that is reflected off of a semiconductor, thermally sensitive diaphragm, with the diaphragm deflected in response to arterial pulsation. The diaphragm""s thermal properties affect how its surface is deflected. Such thermal properties are associated with calibration coefficients which are used for mapping measured deflections into mmHg blood pressure values. The calibration procedure requires taking such thermal properties into consideration, including thermal heating of the diaphragm. Additional calibration considerations are optimum vs. non-optimum applanation state of the underlying artery, compensation for deformable and a nondeformable portions of the diaphragm, so that calibration coefficients can be obtained to map measured sensor output signal into blood pressure.
The present invention is believed to be a substantial improvement over the type of sensors proposed in the prior art. The sensor itself does not depend on thermal considerations. The diaphragm or reflective surface in the present sensor is responsive to any input stress on its surface. Furthermore, a priori knowledge of the exact applanation state is not needed for proper calibration.
Additionally, the sensor is calibrated against a standard conventional air cuff for measuring blood pressure. The calibration procedure automatically compensates for variability that is inherent in patient anatomy and physiological parameters, such as body weight, size, skin thickness, arterial depth, arterial wall rigidity and compliance, body fat, etc. When the sensor is calibrated against known blood pressure (such as using an air-cuff system) all such detailed variables are individually and collectively integrated and linearized in the process of calibrating the sensor. In other words our calibration process is customized to the individual patient anatomy. Accordingly, the sensor and method of the invention produces more accurate results.
The ""027 patent describes a set of detectors which are arranged in a single dimensional row. Image processing techniques are not particularly applicable in the format of arrangement of the detectors. In contrast, the sensor and method of the present invention uses a two-dimensional array of photo-sensitve elements which is cabable of producing a digitized two-dimensional image of the underlying skin surface variations due to pulsation. The number and density of elements are significantly higher. Accordingly, the array produces an image that can be processed using image processing techniques, including image transformation algorithms to detect translation or rotation of the sensor. Image processing methods can also be used for filtering, calibrating, tracking, and error-correcting the output of the sensor.
The ""027 patent requires a mechanical assembly to provide a means for mechanically pushing the sensor onto the surface of skin tissue, and adjusting the force used for obtaining optimal artery applanation. The present invention does not require the need for such stress-sensing mechanical assembly for proper positioning and adjustment to achieve optimum applanation of the artery. The sensor does require a measurable hold down pressure to be applied on the sensor to produce measurable results for calibration purposes. The hold down pressure can be produced by mounting the sensor to a wrist watch band for example. Furthermore, the sensor and inventive method provide for compensating for changes in the hold down pressure between initial or calibration values of hold down pressure and values of hold down pressure later on when blood pressure data is obtained.
The present invention thus provides a convenient, non-obtrusive, wearable device for accurate and reliable continuous noninvasive blood pressure (NIBP) monitoring of an individual either as a standalone unit, or in conjunction with an in-home or hospital wireless base unit and associated monitor. The blood pressure data can be visually displayed to the user on the device itself or can be wirelessly transmitted to the base unit. The base unit can be coupled to a computer for collecting, displaying, and analyzing data, or coupled to a wireline interface to an external monitoring station. The sensor can also be used to monitor other physiologic parameters in addition to blood pressure, such as blood flow, pulse rate, pulse pressure, and arterial compliance.
In a first aspect of the invention, a sensor assembly for acquiring blood pressure data from a patient is provided. The sensor includes a housing adapted to be placed adjacent to the patient""s body, such as at the wrist, and a strap or similar means for applying a hold down force for the sensor in a location where the blood pressure data is to be acquired during use of the sensor assembly. The sensor also includes a source of photo-radiation, which in preferred embodiment takes the form of one or more coherent light sources, such as laser diodes. The laser diodes may be arranged in a two dimensional array in one possible embodiment. The sensor also includes a two-dimensional, flexible reflective surface. The reflective surface may take the form of a reflective coating applied to a polymeric membrane. The reflective surface is nominally positioned relative to the radiation source such that the radiation travels in a direction normal to the reflective surface. The reflective surface is placed adjacent to the location on the patient where the blood pressure data is to be acquired. A hold down pressure sensor, preferably in the form of a strain gauge arranged as a flexible membrane or diaphragm, is also incorporated into the sensor.
Radiation from the source is reflected off of the reflective surface onto a two-dimensional array of photo-detectors. The array of photo-detectors is nominally placed in the optical path of the radiation source, but they do not block all the radiation; rather they are spaced from one another to allow incident radiation from the source to pass in between the detectors and impinge upon the reflective surface at an angle that is normal to the reflective surface. Systolic and diastolic blood pressure fluctuations in the patient are translated into deflections of the patient""s skin. These deflections cause corresponding deflections in the two dimensional reflective surface. The associated movement of the flexible reflective surface due to blood pulsation causes scattering patterns to be detected by the array of photo-detectors. These scattering patterns are processed either in the sensor assembly or in a remote processing unit into useful blood pressure data for the patient.
In a preferred embodiment, the blood pressure sensor is calibrated against known blood pressure data and scattering patterns obtained while the known blood pressure is obtained at a known hold down pressure. During data acquisition, scattering patterns (i.e., output signals from the photo-detectors) are linearly scaled to the calibrated values of signal output and hold down pressure. Thus, the calibration is patient-specific and thereby more accurate than prior art calibration techniques for optical sensors.
The blood pressure sensor may also include a wireless transceiver for sending blood pressure data to a base unit and for receiving configuration or data acquisition commands from the base unit. The sensor may also include a microcontroller and Digital Signal Processor (DSP) or other type of computing platform and a memory storing a set of instructions. The computing platform in the sensor is responsive to commands from the base unit, such as start and stop data acquisition. Together, the blood pressure sensor and the base unit comprise an optical, noninvasive wireless blood pressure data acquisition system.
In another aspect, a method is provided for obtaining blood pressure data from a patient using an optical blood pressure sensor. The optical blood pressure sensor has a two-dimensional array of photo-detectors detecting scattering patterns from a reflective surface placed against the surface of the patient. The method comprises the steps of placing the optical blood pressure sensor against the patient""s body at a location where blood pressure data is to be obtained, and simultaneously measuring the patient""s blood pressure with a second blood pressure device (which can be a conventional sphygmomanometer) and measuring the hold down force of the blood pressure sensor against the patient""s body. Output signals from the array of photo-detectors are obtained. The output signals are calibrated against the measured blood pressure and hold down force, and calibration data is stored in a memory. Then, when blood pressure data is obtained, the sensor obtains output signals from the array of photo-detectors during a blood pressure data acquisition period. The hold down force is also obtained. The output signals from the detectors are scaled to the previous calibration data and hold down force data to thereby obtain blood pressure data.
A method of calibrating a noninvasive optical blood pressure sensor is also provided. The method comprises the steps of placing the optical blood pressure sensor against the patient""s body at a location where blood pressure data is to be obtained, and simultaneously measuring the patient""s blood pressure with a second blood pressure device and measuring the hold down force of the blood pressure sensor against the patient""s body. Output signals from the array of photo-detectors are obtained. The output signals are calibrated against the measured blood pressure and hold down force, and calibration data is stored in a memory.
Further details on these and other features of the invention will be described in the following detailed description of a presently preferred embodiment of the invention.